Oil contains lots of stuff. Cool stuff.. like detergents and additives.. and bad stuff, like Suphur, Ash, Potassium (SAPs)
the chains of molecules need to stay together for the life of the oil. The sheering properties of an engine, (pulling the molecules apart for want of a better word), under both super hot temps, ice cold temperatures, while potentially having small amount sof combustion gases mixed into them, makes engine oil life HELL ON EARTH.
Imagine this - 1 engine with slight wear on one set of piston rings, meaning that a tiny tiny amount of exhaust gas is squished through the rings down into the sump on every ignition/expansion cycle of the engine
1 engine doesnt have that, but runs in freezing temperatures, gets super hot.. and every day splashes through puddles of ice water, cooling the sump over and over.
I cannot tell you what this does to the oil, but I am sure it's DIFFERENT for both. And not pretty.
Then another engine that revs to 7000rpm every day.. and another that ticks over in traffic ... but undergoes lots of stop start because of the Stop/Start system. When an egine stops turning, the oil pressure in the bearings that support the crank shaft is squished out... over and over. Same for the cam shafts.
and then a fourth engine that never gets warm. It turns on, drives 3 miles switches off.. and 7 hours later turns on drives three miles and turns off.
Cheap oil, ala American cars of old, where you could get the oil changed every 3750 miles or allegendly lose your warranty... had cheap oil. badly refined bulk rubbish. But it came free with a steak meal (yes.. American drive through oil service and dinner)
Mobil, a LOOOONG time ago cameout with Mobil1 and I ran it in my rally cars. It came out looking/feeling like brand new ...unlike the old 10w40 that was black after 3 events. Mobile tried and failed to hold the legal rights for "sysnthetic". It was never gonna stick.
But they did a great thing in my opinion, they got top grade oil to the masses. Bloody expenvie but very good.
Look up what spec you engine needs and stick to it. Dn't run fully synthetic 0w30 in a classic. Don't run 10w40 is a modern BMW.
ALWAYS change the filter with the oil
ALWAYS buy A QUALITY FILTER - the hydraulic pressure bearing into the centre of an oil filter can crush a weak filter core.
And dip it often